Polyphenols in Dioscorea Steriscus and Plecranthus Esculentus Tuber Extracts and Effect of the Extracts on Oxidative Stability of Sunflower Oil
Edible oils such as sunflower oil are important dietary components that are widely used in food processing and preparation. The ease of oxidation of edible oils has made their safe storage a huge problem to food processors and handlers. Oxidative deterioration of edible oils lowers their quality and contributes to a number of human health risks. Improving the safety of edible oil based products through the prevention of oil oxidation is very crucial in the food and allied industries. Studies focusing on plant polyphenols as a natural source of preservatives to curb oil and food oxidation have widely escalated. Even though synthetic antioxidants are the commonly used preservatives for edible oils, plant derived natural antioxidants are considered to be safer and cheaper. Dioscorea steriscus Burkill and Plecranthus esculentus N.E.Br are tuberous traditional medicinal plants indigenous to Africa. Despite being potential sources of potent medicinal compounds and antioxidants, D. steriscus and P. esculentus have remained little studied and poorly understood plants. The major aim of this study was to investigate the preservative potential of polyphenols extracted from D. steriscus and P. esculentus plant tubers on sunflower oil stored under different conditions. The tubers of the two plants from Zimbabwe were collected, extracted, screened and analysed for polyphenols using conventional techniques. The preservative potential of the tuber extracts against oxidative deterioration of sunflower oil under different conditions was investigated. The yields for polyphenol extraction were found to be 6.32 % and 6.38 % for peeled and unpeeled D. steriscus and 5.44 % and 5.49 % for peeled and unpeeled P. esculentus tubers respectively. Polyphenol screening assays on D. steriscus and P. esculentus tuber extracts showed the presence of flavonoids, phenolic acids, tannins, anthocyanins and lignans. The ultraviolet-visible (UV-Vis) spectrophotometric scanning of D. steriscus and P. esculentus tuber extracts depicted the presence of polyphenolic organic chromophores. Fourier-transform infrared (FTIR) spectroscopic analysis of both D. steriscus and P. esculentus tuber extracts confirmed the presence of organic compounds with hydroxyl, aromatic, amine, carboxyl, carbonyl, acyl and alkyl groups. The high performance liquid chromatography-electrospray ionization-mass spectrometric (HPLC-ESI-MS) analysis confirmed the presence of kaempferol and vanillic acid in both D. steriscus and P. esculentus tuber extracts. Both D. steriscus and P. esculentus polyphenol extracts showed good preservative potential against oxidation of sunflower oil under accelerated oxidative conditions and room storage conditions with peroxide values ranging from 7.27 ±0.02 - 24.77 ±0.04 mEq/kg and UV-Vis absorbances at 232 nm as indicators of the concentrations of conjugated dienes ranging between 0.064 ±0.001 and 1.37 ±0.003. One-way ANOVA (α =0.05) revealed no significant difference between these results and those of the reference antioxidant, butylated hydroxytoluene (BHT) under accelerated oxidative conditions and room storage conditions. From these results, it can be concluded that both D. steriscus and P. esculentus consist of polyphenols that can be potential source of natural antioxidants for preservation of vegetable oils. This study is expected to raise interest among researchers, farmers and agronomists on the two plants, hence promoting the preservation and cultivation of the plants.